1. Field of the Invention
The invention relates to the field of manufacture of microelectronics fabrications. More particularly the invention relates to the field of monitoring and detecting contamination in silicon integrated circuit microelectronics fabrication processes.
2. Description of the Related Art
Fabrication of silicon integrated circuit microelectronics devices requires the employment of various chemical processes. A typical microelectronics fabrication is composed of many different materials laminated together, some of which are patterned employing masking techniques and subtractive etching methods employing both wet and dry chemical environments. Of particular interest are chemical systems for etching silicon containing dielectric layers in a controlled fashion. Silicon oxide layers, for example, may often be required to be removed by etching from silicon substrates at a known rate and without deleterious effects on the silicon substrate.
One of the principal factors involved in obtaining the proper etch rate of a reagent is the presence or absence of certain impurities. Hence a particular concern is the assurance of purity of reagents. In the case of etching silicon oxide layers on silicon substrates, the etching performance of a particular acid etch bath known as buffered oxide etch (BOE) is greatly affected by the reagent concentration and temperature. Although satisfactory performance is obtained from BOE acid etch bath when proper concentration and temperature are maintained, the use of BOE acid etch bath for etch removal of silicon oxide layers from silicon substrates before metallization is not without problems.
Etching baths are constantly monitored to assure that they are functioning properly and providing the required etch rate. It is a common practice to treat BOE acid etch baths with hydrogen peroxide (H2O2) to control certain impurity substances which may cause deviations from the specified etch rate. Since hydrogen peroxide may act as an oxidizing agent in acid baths, any excess of this material may itself act as a particular contaminant which could result in yield loss of microelectronics fabrications at Wafer Acceptance Test (WAT) final test. A lengthy period of time is thus required to determine if contamination was present by yield loss data, due to the large number of processes. Some form of current real-time process monitoring is desirable to determine if oxidizing contamination by substances such as, for example, hydrogen peroxide has occurred in the BOE acid etch bath.
It is therefore towards the goal of assuring the freedom of acid etching baths employed within microelectronics fabrication processes from impurities such as oxidizing substances that the present invention is particularly directed.
Various methods and devices have been proposed to monitor the condition of manufacturing equipment and chemical reagent solutions employed in microelectronics fabrication with respect to contamination by deleterious substances.
For example, Golden, in U.S. Pat. No. 4,668,330, discloses a method for employing a test wafer to monitor furnace contamination from heavy metal impurities. The method employs exposure of silicon substrates whose degree of oxygen induced stacking faults is a measure of the extent of contamination by heavy metal elements to the furnace environment. The observation of stacking faults by selective etching of the monitor wafers.
Further, Falster et al., in U.S. Pat. No. 5,418,172, disclose a method employing a monitor wafer to measure transition metals contamination in or on equipment and fluids employed in fabrication or handling of silicon wafers employed in silicon integrated circuits. The method employs the exposure of monitor silicon wafers which have a measured minority carrier lifetime of greater than about 250 microseconds to a given process employing equipment or fluids. As part of or in addition to the process, the monitor wafer is exposed to a temperature of at least 600 degrees centigrade and the minority carrier lifetime re-measured to ascertain the amount of transition metals picked up by the monitor silicon wafer.
Still further, Carpio, in U.S. Pat. No. 5,439,569, discloses a method for measuring H2O2 concentration in chemical baths used for semiconductor processing. The method employs potentiometric and amperometric measurements whose values are related to the amounts of ammonia or water present in the hydrogen peroxide baths.
Finally, Barbee et al., in U.S. Pat. No. 5,573,623 and U.S. Pat. No. 5,338,390, disclose a method employing contactless electrodes to determine the end point of etching or other similar process where an abrupt phase change accompanies the completion of the process. The electrodes are toroidal coils positioned adjacent to the objects being processed, and are supplied with an AC signal which is monitored. When the process being monitored is completed, there is an abrupt change in the transmission characteristics of the AC signal which is sensed as the end point of the process.
Desirable in the art of silicon integrated circuit microelectronics fabrication are additional methods for monitoring the amount of contamination present in acid baths for etching silicon oxide layers on silicon substrates.
It is towards this goal that the present invention is generally and specifically directed.
A first object of the present invention is to provide a method to detect the presence of contamination in a reagent solution employed in the manufacture of silicon microelectronics fabrications.
A second object of the present invention is to provide as a means in accord with the first object of the present invention a monitor silicon substrate to implement the detection and monitoring of contamination in an buffered oxide etch (BOE) acid bath employed to etch silicon oxide layers formed upon silicon substrates in the manufacture of silicon integrated circuit microelectronics fabrications.
A third object of the present invention is to provide a method in accord with the first object of the present invention and/or the second object of the present invention, to fabricate a monitor silicon substrate employed as a means to detect and monitor the concentration of oxidizing contamination in a 50:1 BOE acid etch bath employed to etch silicon oxide layers formed upon silicon substrates in the manufacture of silicon integrated circuit microelectronics fabrications.
A fourth object of the present invention is to provide a method, in accord with the first object of the present invention, the second object of the present invention and the third object of the present invention, which is readily commercially implemented.
In accord with the objects of the present invention, there is provided a method for detecting and monitoring the concentration of oxidizing contamination in an acid bath employing a monitor silicon substrate, and a method for fabrication of said silicon substrate monitor. To practice the invention, there is first provided a surface doped silicon substrate whose electrical sheet resistance is measured. Upon the silicon substrate is formed a silicon oxide dielectric layer to form a monitor silicon substrate. The monitor silicon substrate is then placed in the buffered oxide etch (BOE) acid etch bath to be tested for a period of time. Then the monitor silicon substrate is removed from the etch bath and the electrical sheet resistance measured again. An increase in the electrical sheet resistance of the silicon substrate greater than the standard deviation is a measure of the presence of oxidizing contamination in the acid bath.
The present invention provides a method for monitoring an acid etching bath employed to etch a silicon oxide layers completely from a silicon substrate upon which the silicon oxide layer is formed. A means for implementing the method employs a silicon substrate fabricated in accord with the following procedure: (1) A layer of silicon oxide is formed upon a P-type silicon wafer of between 15 and 25 ohm-centimeter resistivity employing thermal oxidation to a thickness of 250 angstroms; (2) an ion implant employing AS75 ions is performed to provide a doped surface layer; (3) a rapid thermal anneal is performed at 1100 degrees centigrade.
The present invention provides a method for measuring the extent of oxidizing contamination in a buffered oxide etch (BOE) acid etching bath of concentration ratio of which is employed to remove completely all silicon oxide surface layers prior to further processing.
The present invention employs methods and materials as are known in the art of silicon integrated circuit microelectronics fabrication, but in a novel and original arrangement. Therefore the present invention is readily commercially implemented.